This invention relates to an infrared radiation detector comprising a pyroelectric element sandwiched between two electrodes. The pyroelectric element is maintained in spaced relation to a supporting surface by support means. The detector further comprises a circuit arrangement with at least one circuit element connected electrically to one of the electrodes.
Infrared detectors can be used, for example, in intruder alarms. It is because the intruder is a moving source of infrared radiation that his presence can be detected. The infrared radiation emitted by the intruder is converted by the detector into an electric signal which can be used to actuate an alarm.
In order that the detector is well isolated from detrimental environmental effects and for maximum performance at low frequencies, it is common to mount the element so that it is spaced apart from a supporting surface. This minimizes heat transfer from other parts of the detector to the pyroelectric element and lengthens the thermal time constant.
A basic infrared detector may comprise a single pyroelectric element sandwiched between two electrodes. As the pyroelectric element has a relatively high impedance it is customary to include a circuit element in the form of a field effect transistor (hereinafter referred to as an F E T), which acts as an amplifier stage or impedance matching means. The gate of the FET is connected to one of the two electrodes.
In more sophisticated detectors, the circuitry (which is located in the proximity of and in the same envelope as the pyroelectric element) may be more complex. For example, United Kingdom patent GB No. 1,580,403 describes a detector which also includes a nonlinear network which protects the gate of the FET from excessive voltages resulting from large or sudden changes in input radiation.
As described in that patent, the pyroelectric element is maintained in spaced relation to the supporting surface of a header by a U-shaped plinth member. The circuit elements which are located in the proximity of the pyroelectric element are unencapsulated chips. The various electrical connections to these chips are made by bonded wires. Unfortunately, the technique of wire bonding is both expensive and difficult to perform accurately so that the yield of usefully bonded detectors can be undesirably restricted.
Also, there is another problem which results from the use of unencapsulated chips, namely the difficulty of performing certain desirable tests on the chip, for example measurement of low leakage current and low voltage noise of the circuit elements. These tests can be carried out after the chips have been included in the detector, however it is then too late to identify an unsuitable chip until the detector is virtually complete. It can be expensive to test the chips at this late stage because an unsuitable chip then results in the rejection not only of the chip itself, but also of the whole detector comprising that chip.